Does anyone have any advice/tips on performing an in place upgrade from a Druid based system to the current FreePBX?
You’ve provided almost no details so you are making it hard to answer. First off use the backup option for your current version of FreePBX and make a complete backup of all settings. Depending on how old your version is you might want to also backup other important directories that might not be backed up. Those would include any place you have customized any scripts, tftp directory, etc.
Upgrading from some versions to others might require you to use switches to to make sure that things are upgraded. But without the details of your system we can only give very generic info.